Auto flashes are handy when used on their own. They use a thyristor to shut off the flash once the exposure has been deemed sufficient by the flash.

When using multiple flashes in conjunction, however, it is best to set their power levels manually, i.e., not let the flash decide which output level to use.

Many Auto flashes do not allow the setting of the power level but here's a trick to get at least one manual level:

If one blinds the photo diode with which the flash measures the exposure (say with a opaque tape), the flash will always go full power with respect to a particular setting. Depending on the level chosen, this will result in a consistent flash output, corresponding to a manual power setting.

Example:
The Vivitar 2500 has four settings:

• M (manual) = full power
• blue (auto setting 1) = up to full power
• red (auto setting 2) = up to ~1/3 power
• triangle (autowinder mode) = 1/8 power

So with this flash one can achieve three output levels: full, ~1/3 (requires blinding of the photo diode), and 1/8.

I suspect that one of the auto setting of an auto flash will always go up to full power. I wouldn't make sense if the flash weren't trying to give "its best".

P.S.: When using older flashes, always make sure that their trigger voltage is not too high when using them on-camera.

[stu49]

You can also cover the sensor with sticky paper circles to up the power output (more circles, more power) ;-) handy if the flash only has 1 auto mode.